Controller for electric power steering device

ABSTRACT

A control apparatus of an electric power steering apparatus is provided in which a nonlinear element of a motor model of-the electric power steering apparatus is compensated beforehand to linearize the motor model and the back electromotive force of the motor is computed to compensate the back electromotive force for a control loop, and conduct back electromotive force compensation with no lag in order to realize a control apparatus of an electric power steering apparatus with less control error, stable controllability, small motor output torque ripple, good wheel steering feeling, and less motor noise.

TECHNICAL FIELD

The present invention relates to a control apparatus of an electricpower steering apparatus in which steering auxiliary power by a motor isgiven to a steering system of automobiles and vehicles, and to a controlapparatus of an electric power steering apparatus in which nonlinearelements of a motor which are the targets for control are separated fromlinear element to allow control with less error.

BACKGROUND ART

An electric power steering apparatus which energizes a steeringapparatus of automobiles and vehicles in auxiliary power by motor torqueenergizes a steering shaft or a rack shaft with motor driving force inauxiliary power by a transmission mechanism such as a gear or a beltthrough a reduction gear. Such a conventional electric power steeringapparatus is devised variously on control in a control apparatus of theelectric power steering apparatus which energizes a steering apparatusby assist torque (steering auxiliary torque) so that a driver cansmoothly conduct steering under any situations such as in high or lowspeed drive, or in running straight or at a curve, or in parking.

First, before introducing examples of specific conventional controlmethods, control over the electric power steering apparatus will bedescribed in general. When the relationship between voltage and currentof a motor, which are main control targets of the electric powersteering apparatus, is expressed in an equation, it can be expressed byEquation (1).V=EMF+(R+s·L)·I   (1)Here, V is motor terminal voltage, EMF is motor back electromotiveforce, I is motor winding current, R is a motor winding resistance, andL is a motor winding inductance. S is a Laplace variable, whichexpresses d/dt. In addition, EMF is expressed by the following Equation(2).EMF=Ke·ω  (2)Here, Ke is a back electromotive force constant, and ω is rotor angularvelocity.

(R+s·L)·I, which is the second term of Equation (1), is an electricelement and has linearity. However, EMF of the first term is generatedby the motor angular velocity ω, and greatly affected by nonlinearelements such as external forces from tires, and inertia and friction ofmechanical elements of the electric power steering apparatus. Generally,the nonlinear element is hard to be controlled.

Here, feedback control (hereinafter, it is denoted as FB control), whichis a typical control method, will be described briefly. Generally, FBcontrol is that a target value, which is a certain control target, iscontrolled so as to match with a certain reference value. Typically, thedifference between the target value and the reference value is inputtedto a proportional integral circuit (hereinafter, it is denoted as a PIcircuit), for example, for control. Then, the input of the PI circuit isa signal which includes all the mixed influences of variation of thereference value, disturbance and noise to the target value, fluctuationsin parameters, and the like. Regardless how and which element among themexerts influence upon the control, it is such a quite simple controlthat whether the output matches with the reference value is determinedand when there is no matching between them correction operation isconducted. Therefore, this pure FB control performs the correctionoperation only when an error exists so that the output is variedunstably near the reference value, which appears as a torque ripple ofmotor output in the electric power steering apparatus. A great torqueripple causes a problem that a driver feels uncomfortable in steering orwith increased motor noise.

Here, there is an apparatus described in JP-A-2002-249061 as an exampleof the control apparatus using FB control (hereinafter, Patent Reference1). The description will be described with reference to FIG. 5. Based oninputted car speed and steering torque, a current command value I_(ref)is computed in a target current deciding section 120. More specifically,a steering torque detector 101 connected to a torque sensor, not shown,detects steering torque, a phase compensator 108 compensates a phaselag, and then its output is inputted to a steering torque controller102. Furthermore, a car speed signal detected by a car speed detector114 is inputted to the steering torque controller 102, and based on theboth inputs, a torque value that assists steering torque generated bymanipulation of a steering wheel by a driver is determined. Then, thetorque value to be assisted is inputted to a motor current decider 107to decide target current I_(ref).

Subsequently, actual current I_(act) of the output of a motor actuator109 is detected by a motor current detector 111 and is fed back to asubtraction circuit 113 to which the target current I_(ref) is inputted.Error between the target current I_(ref) and the actual current I_(act)is computed, and it is inputted to a first current controller 103.Basically, output V_(dFB) of the first current controller 103 drives themotor actuator 109 to control a motor 110. However, an auxiliary signaland an auxiliary control loop, described below, are added in order tosmoothly conduct steering. First, as the auxiliary signal, disturbancevoltage V_(dist1) and disturbance voltage V_(dist2), and backelectromotive force V_(b) are compensated. Moreover, a disturbancevoltage estimation observer 115 observes whether the motor actuator 109makes output according to V_(ref) that is a command value. The totaldisturbance voltage is (V_(dist1)+V_(sist2)+V_(b)), but the backelectromotive force V_(b) is proportional to the steering speed, about 3Hz at the maximum, whereas the disturbance voltage caused by brushvibrations and commutation ripples is 20 to 200 Hz. Thus, a highpassfilter 116 is used to remove the back electromotive force V_(b), and toextract only the disturbance voltage V_(dist). The extracted disturbancevoltage V_(dist) is inputted to a second current controller 105, and itsoutput is added to V_(dFB) by an adding circuit 112 a, to compute motordrive command voltage V_(ref). The motor drive command voltage V_(ref)is different from V_(ref), a basic control described above; it is amotor drive command voltage V_(ref) that is corrected including thedisturbance voltage which, compared with the basic control, enables tocope with various steering situations described above so as to smoothlymanipulate the steering wheel. Accordingly, when the control apparatusin the configuration like this attempts to meet various steeringsituations, complicated control elements such as the highpass filter 116and the second current controller 105 need to be added, resulting in acomplicated control circuit.

Furthermore, Japanese Patent No. 2949183 (hereinafter, it is calledPatent Reference 2) discloses an example of the conventional controlapparatus of the electric power steering apparatus which compensates theback electromotive force described above. It will be described withreference to FIG. 14. In the drawing, control is carried out so thatcommand value I_(r) computed in a torque sensor 100 mounted on theelectric power steering apparatus is inputted to a control circuit 101,and a PWM control circuit 12 controls an inverter circuit 13 based onthe command of the control circuit 101 to drive a motor 14. In thecontrol circuit 101, motor current I_(f) detected by a current detectioncircuit 15 is fed back, error between the command value I_(r) and themotor current I_(f) is calculated, and the error is inputted to a PIcircuit to compute the command value V_(d). In the basic control method,the command value V_(d) is split into three phases (a-phase, b-phase,and c-phase) to be command values for the PWM control circuit 12.

Moreover, in the control circuit 101, ω is determined from revolutions Ndetected by a rotational speed sensor 102 mounted on the motor 14, backelectromotive force E is detected from Equation (2), the backelectromotive force E is added to the command value V_(d) to compute anew command value V_(m), and the motor 14 is controlled based on theV_(m). The compensation of the back electromotive force E allowssmoother steering than the basic control method.

However, in the case of the scheme in which the motor revolutions aredetected by using an encoder to compute the back electromotive force asshown in Patent Reference 2, the operation period of revolutions is setlonger than the current control period in order to increase theresolution of revolutions to accurately compute the back electromotiveforce even in low-speed rotation. This becomes a factor to increase alag in high-speed rotation. When the back electromotive force is smalland the effect of the back electromotive force compensation is small asin low-speed rotation, the lag problem has small influence upon the backelectromotive force compensation, but the lag causes a problem that theeffect of the back electromotive force compensation is reduced when theback electromotive force is great as in high-speed rotation. Morespecifically, in high speed rotation, the back electromotive forcecompensation without delay is desirable.

The invention has been made in view of the circumstances. The inventionrelates to a control apparatus of an electric power steering apparatus.A first object of the invention is to compensate a nonlinear element ofa motor of the electric power steering apparatus beforehand to linearizethe motor. Furthermore, a second object is to conduct back electromotiveforce compensation with no lag by a control apparatus in which the backelectromotive force of a motor is computed to compensate the backelectromotive force for a control loop. By achieving the objects, acontrol apparatus of an electric power steering apparatus with lesscontrol error, stable controllability, small motor output torque ripple,a good steering feeling, and less motor noise is provided.

DISCLOSURE OF THE INVENTION

The invention relates to a control apparatus of an electric powersteering apparatus in which steering auxiliary power by a motor is givento a steering system of a vehicle, the first object of the invention isachieved by including:

a motor drive circuit which drives the motor;

a current control circuit (11) which computes a first voltage commandvalue that is a control command to the motor drive circuit;

a back electromotive force computation circuit (17) which computes aback electromotive force value of the motor based on Output voltage andoutput current of the motor drive circuit; and

an adding circuit (18) which adds the back electromotive force to thefirst voltage command value and computes a second voltage command valuethat is a new control command to the motor drive circuit.

Furthermore, the first object is effectively achieved in a manner inwhich:

a second adding circuit (23) is disposed between the current controlcircuit (11) and the adding circuit (18);

output of the current control circuit (11) is input to the second addingcircuit (23), and output of the second adding circuit (23) is input tothe adding circuit (18);

a disturbance observer circuit (19) is disposed which has output of thesecond adding circuit (23) and the output of the motor drive circuit asinput; and

a disturbance value which is output of the disturbance observer circuit(19) is inputted to the second adding circuit (23), added to the firstvoltage command value, and inputted to the adding circuit (18).

Moreover, the first object is effectively achieved in a manner in which:

the disturbance value is a difference between a value obtained bymultiplying the input value of the adding circuit (18) by a transferfunction and a value obtained by multiplying an output value of themotor drive circuit is multiplied by the transfer function. Besides, thefirst object is effectively achieved in a case that:

the current control circuit (11) is a feed forward control or feedbackcontrol.

Furthermore, the second object of the invention is effectively achievedby including:

a motor drive circuit which drives a motor;

a first back electromotive force computation circuit which computes backelectromotive force (EMF1) of the motor based on the output voltage andoutput current of the motor drive circuit;

a phase computation circuit which computes an electrical angle (θ) andan angular velocity (ω) based on the back electromotive force (EMF1);

an adjustment circuit which computes an adjusted electrical angle (θ+Δθ)where a phase lag (Δθ) is compensated by the angular velocity (ω); and

a second back electromotive force computation circuit which computes anadjusted back electromotive force (EMF2) based on the adjustedelectrical angle (θ+Δθ).

Moreover, the second object of the invention is more effectivelyachieved in a manner in which:

a current control circuit which computes a command value (V_(ref)) todrive-control the motor based on a steering torque command value(T_(ref)) to the motor,

wherein the motor is controlled based on a command value (V_(ref)+EMF2)where the adjusted back electromotive force (EMF2) is added to thecommand value (V_(ref)).

Besides, the second object of the invention is achieved by including:

a current control circuit which computes a command value (V_(ref)) todrive control the motor based on a steering torque command value(T_(ref)) to the motor;

a motor drive circuit which drives the motor;

a first back electromotive force computation circuit which computes backelectromotive force (EMF1) of the motor based on the output voltage andoutput current of the motor drive circuit; and

a correction circuit which computes a corrected back electromotive force(K·EMF1) where the back electromotive force (EMF1) is multiplied by aset value (K),

wherein the motor is drive-controlled based on a value (V_(ref)+K·EMF1)where the command value (V_(ref)) is added to the corrected backelectromotive force (K·EMF1). Furthermore, the second object is achievedin a case that:

the current control circuit is a feed forward control circuit or afeedback control circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is explanatory diagrams illustrative of linearization of a motor.

FIG. 2 is a control block diagram illustrating an electric powersteering apparatus in which a first aspect of the invention is appliedto a feed forward control.

FIG. 3 is a control block diagram illustrating an electric powersteering apparatus in which a second aspect of the invention is appliedto a feed forward control.

FIG. 4 is a control block diagram illustrating an electric powersteering apparatus in which the first and second aspects of theinvention are applied to a feedback control.

FIG. 5 is a control block diagram illustrating an electric powersteering apparatus to which conventional feedback control is applied.

FIG. 6 is a control block diagram illustrating a control apparatus usinga feed forward control with a back electromotive force whose lag iscompensated, as a third aspect of the invention.

FIG. 7 is a block diagram illustrating the detail of a first backelectromotive force computation circuit and a phase computation circuit.

FIG. 8 is a block diagram illustrating the detail of an adjustmentcircuit which compensates a phase lag and a second back electromotiveforce computation circuit.

FIG. 9 is a block diagram illustrating a computing section of the backelectromotive force, targeting a three-phase motor.

FIG. 10 is a control block diagram illustrating a control apparatususing a feedback control with the back electromotive force whose lag iscompensated, as the third aspect of the invention.

FIG. 11 is diagrams illustrative of the principle of lag compensation incomputing the back electromotive force by gain adjustment as a fourthaspect of the invention.

FIG. 12 is a block diagram illustrating lag compensation in computingthe back electromotive force by gain adjustment as the fourth aspect ofthe invention.

FIG. 13 is a diagram illustrating effects of the third and fourthaspects of the invention by simulation.

FIG. 14 is a control block diagram illustrating a conventional controlapparatus by back electromotive force compensation using a conventionalrotational speed sensor with no lag compensation.

BEST MODE FOR CARRYING OUT THE INVENTION

First, a theoretical basis of the invention is shown, and thenembodiments will be described.

A point of the invention to achieve the first object of the invention isto separate nonlinear elements from a motor, which is a control target,to linearize the motor. The nonlinear elements of the motor, are roughlycategorized into two elements: a motor back electromotive force andother nonlinear elements such as noise. Then, since the backelectromotive force, which is a state variable that may influence uponoutput, can be computed beforehand, it is computed by a scheme describedlater. Then, for the nonlinear elements of the other category, anobserver (state observer), described later, can be used when a statevariable that is considered to have influence upon output is unknownbeforehand or when the influence of disturbance and noise is removed.Thus, the observer is used to extract the nonlinear elements of theother category.

First, a first aspect of the invention that achieves the first object ofthe invention, in which the back electromotive force is compensated tolinearize the motor, will be theoretically described with reference toFIG. 1.

FIG. 1(A) is a diagram illustrating an actual motor. It corresponds toEquations (1) and (2) described above when expressed by equations. Then,as the back electromotive force EMF=Ke·ω is a nonlinear element asdescribed in Equations (1) and (2), the motor model in FIG. 1(A) is anonlinear element. To linearize this motor model, as shown in FIG. 1(B),the EMF estimation value (computed value) is estimated, and is added tothe voltage command value V_(ref). Then, the influence of the backelectromotive force EMF of the actual motor can be canceled, and themotor model can be linearized as 1/(R+s·L).

Next, a specific scheme to estimate (compute) the back electromotiveforce will be described below. When the motor is expressed by anequation, it is as shown in Equation (1). When this is modified tocalculate the back electromotive force EMF, it is expressed as thefollowing Equation (3):EMF=V−(R+s·L)·I   (3)where, V is motor voltage, I is motor current, EMF is the motor backelectromotive force, R is a motor resistance value, L is a motorinductance value, and s is a Laplace variable.

When the back electromotive force EMF is fully compensated, Equation (3)can be expressed as Equation (4):I/V=1/(Rn+s·Ln)=Pn   (4)where, Rn is a nominal motor resistance, Ln is a nominal motorinductance, and Pn is a nominal motor model.

For a property of Equation (4), Equation (4) is a linear equation. WhenEquation (4) is modified as follows,V=I·(Rn+s·Ln)=I·Pn ⁻¹   (5)it is written as a control target:V _(ref) =I _(ref)·(Rn+s·Ln)=I _(ref) ·Pn ⁻¹   (6)Therefore, the current command value I_(ref) is inputted to directlycompute a first voltage command value V_(ref).

In consideration of a practical problem that noise overlaps with aninput signal, it need to be passed through a lowpass filter(hereinafter, it is denoted as LPF) of the first order lag. That is:$\begin{matrix}\begin{matrix}{V_{ref} = {I_{ref} \cdot {\left( {{Rn} + {s \cdot {Ln}}} \right)/\left( {1 + {s \cdot T}} \right)}}} \\{= {I_{ref} \cdot {{Pn}^{- 1}/\left( {1 + {s \cdot T}} \right)}}}\end{matrix} & (7)\end{matrix}$where, T=½πfc, T is a time constant of LPF, and fc is a cutofffrequency.Thus, Equation (7) is a basic transfer function for FF control. However,since Equation (7) is obtained by modeling only the motor resistance andthe inductance, which are motor electric properties, the conditions tomake at Equation (7) held in actual motor control are:

-   (i) The back electromotive force is fully compensated;-   (ii) The motor model is correct; and-   (iii) There is no imperfect element in the system model such as    detection error or analog-digital conversion noise.

Then, conventionally, FF control which is designed under condition (i),(ii) and (iii) has big control error because of mixed linear elementsand nonlinear elements. Furthermore, even though the disturbance voltageestimation observer 115 shown in FIG. 5, which has been referred inexplanation of the conventional FB control, is used for FF control tocompensate all of (i), (ii) and (i) by the disturbance observer, thevalue of the-back electromotive force exceeds the range that can becompensated by the disturbance observer. Therefore, it cannot becompensated enough to result in remaining a considerable amount ofcontrol error.

As for (i), the invention directly adds a compensation loop for the backelectromotive force to a basic control loop for solution, not by thedisturbance observer. It is difficult for fully, perfectly modeling (ii)due to temperature variation in the motor, for example. Moreover, since(iii) includes the nonlinear elements such as detection error andanalog-digital conversion, (iii) is also difficult for perfectly correctmodeling. Accordingly, the disturbance observer is applied to (ii) and(iii) for compensation.

Described above is the explanation for the theoretical basis of theinvention.

An embodiment of FF control where the back electromotive force iscompensated as the first aspect of the invention will be described indetail with reference to FIG. 2.

First, the circuit configuration of the basic control for the motor willbe described. The torque command value T_(ref) and the Hall sensorsignal S_(hall) are inputted to a current command value circuit 10 tocompute the current command value I_(ref), and input it to a subsequentfeed forward current control circuit 11. For the feed forward currentcontrol circuit, a first order lag circuit with a small time constant,for example is used. A first voltage command value V_(ref), which is theoutput of the current control circuit 11, is inputted to a motor drivecircuit which drive controls a motor 14 through an adding circuit 18.Here, the motor drive circuit is formed of a series circuit of a PWMcontrol circuit 12 and an inverter circuit 13. Note that, the circuitconfiguration of conventional FF control is in which the voltage commandvalue V_(ref) is directly inputted to the motor drive circuit withoutadding a back electromotive force EMF, described later, thereto, by theadding circuit 18. More specifically, in the conventional FF control,all the elements, not only (R+s·L)·I that is the linear element of themotor model expressed by Equation (1), but also the nonlinear elementsuch as the back electromotive force EMF being, or noise are to becontrolled by the voltage command value V_(ref). Thus, it is of thecircuit configuration where control error is likely to be generated.

Then, for the point of the invention, it is important to form theconfiguration in which each of the nonlinear elements is separated andextracted, that is, the elements that may have influence upon output areextracted and compensated beforehand and then the motor model islinearized for control. The nonlinear elements are divided into themotor back electromotive force EMF with great influence and theremaining nonlinear elements including noise and the others to becalculated respectively.

First, in order to compute the back electromotive force EMF, a currentdetection circuit 15 detects the motor current I, a voltage detectioncircuit 16 detects the motor voltage V, and the voltage V and thecurrent I are inputted to a back electromotive force computation circuit17 to compute the back electromotive force EMF of the motor 14. Morespecifically, the back electromotive force EMF is computed by a schemeexpressed by Equation (3). Therefore, in the back electromotive forcecomputation circuit 17, a transfer function circuit 17-1 to which thedetected motor current I is input is disposed, and a subtraction circuit17-2 which computes difference between the output of the transferfunction circuit 17-1 and the motor voltage V passed through an LPFcircuit 17-3 is disposed, and the output of the subtraction circuit 17-2is the back electromotive force EMF determined. Here, the specificfunction of the transfer function circuit 17-1 is (R+s·L)/(1+s·T)expressed by Equation (7). In addition, the denominator (1+s·T) of thetransfer function circuit 17-1 expresses the first order lag circuit ofLPF to remove noise contained in the detected motor current I. Then, itis configured so that the computed back electromotive force EMF is fedback to the adding circuit 18, and the back electromotive force EMF isadded to the first voltage command value V_(ref) which is the output ofthe current control circuit 11 of the conventional FF control circuit tocompute a new, second voltage command value (V_(ref)+EMF).

With the circuit configuration described above, the input of the motordrive circuit is the second voltage command value (V_(ref)+EMF), theoutput of the adding circuit 18, not the first voltage command valueV_(ref), the conventional voltage command value. The motor drive circuitis formed of a series circuit of the PWM control circuit 12 and theinverter circuit 13, and the voltage command value (V_(ref)+EMF) is toexpress the PWM duty ratio of the PWM control circuit 12. The outputcurrent of the inverter circuit 13 PWM controlled by the PWM controlcircuit 12 drives the motor 14 to generate torque.

The operation of the embodiment will be described based on theconfiguration of the embodiment.

First, the current command value I_(ref) is computed in the currentcommand value circuit 10 based on the torque command value T_(ref) andthe Hall sensor signal. The first order lag transfer function, forexample is applied to the current control circuit 11 that is an FFcontrol having the current command value I_(ref) as input, which isbased on the theory of Equation (7). For the time constant and gain ofthe first order lag transfer function, optimum values in the system maybe selected by experiments. Conventionally, since the first voltagecommand value V_(ref), the output of the current control circuit 11, isdirectly inputted to the PWM control circuit 12, it results in greatlyremaining control error because of mixed the linear elements and thenonlinear elements.

The invention is to compensate the back electromotive force EMFbeforehand, which is the nonlinear element having great influence uponoutput of the voltage command value V_(ref), for linearizing thecontrol. In other words, the back electromotive force compensation thatis the condition (i), which is one of the conditions to fully holdEquation (6) is added.

Back electromotive force compensation is computed based on the equationexpressed by Equation (3). That is, based on the motor voltage Vdetected by the voltage detection circuit 16 and the motor current Idetected by the current detection circuit 15 as input, the backelectromotive force EMF is computed by the back electromotive forcedetection circuit 17. More specifically, the detected motor current I isinputted to the transfer function circuit 17-1, difference between the(R+s·L)·I/(1+s·T) which is the output of the transfer function circuit17-1 and the detected motor voltage V is conducted by the subtractioncircuit 17-2, the output of the subtraction circuit 17-2, that is, theoutput of the back electromotive force computation circuit 17 is theback electromotive force EMF determined. In addition, the denominator(1+s·T) of the transfer function circuit 17-1 is intended to do noiseremoval out of the motor current I by the LPF circuit, the first orderlag function.

Subsequently, for the point of the invention, the back electromotiveforce EMF which is the output of the back electromotive forcecomputation circuit 17 is added to the voltage command value V_(ref)which is the output of the current control circuit 11 by the addingcircuit 18, and a new second voltage command value (V_(ref)+EMF) iscomputed. Then, to the PWM control circuit 12 of the motor drivecircuit, not the conventional first voltage command value V_(ref), butthe new second voltage command value (V_(ref)+EMF) which is thecompensated back electromotive force is inputted, and the invertercircuit 13 drives the motor 14 based on the command of the PWM controlcircuit 12.

Consequently, since the motor 14 is controlled in the state that theback electromotive force EMF which is the nonlinear element iscompensated beforehand by the second voltage command value(V_(ref)+EMF), there is an advantage that an electric power steeringapparatus with less control error and small torque ripple can beachieved.

Next, an embodiment of FF control regarding to a second aspect of theinvention will be described with reference to FIG. 3.

The second aspect of the invention relates to a control circuitconfiguration in which the nonlinear elements such as (ii) and (iii)that cannot be estimated beforehand are also compensated to furtherlinearize the motor for more reduced control error. More specifically,the nonlinear elements other than the back electromotive force EMF areextracted by the observer circuit and compensated as similar to the backelectromotive force compensation to further linearize the motor.

In order to fully hold the linearized model expressed by the Equation(6), an embodiment of FF control will be described with reference toFIG. 3, which also incorporates compensation of the remaining conditions(ii) and (iii) in addition to the back electromotive force EMF. Morespecifically, a disturbance observer circuit 19 is added in order tocompensate (ii) and (iii). It monitors whether the motor current I beingthe output of the inverter circuit 13 is correctly outputted withrespect to the input of a second voltage command value.

The output of a second adding circuit 23 and the motor current I areinputted to the disturbance observer circuit 19. In the disturbanceobserver circuit 19, basically, the difference between the output of thesecond adding circuit 23 and the motor current I is calculated, but thedifference between the value obtained by passing the output of thesecond adding circuit 23 through the LPF circuit 20 and the valueobtained by passing the motor current I through the LPF circuit 21 iscalculated in a subtraction circuit 22 as the disturbance value V_(dis).To pass through the LPF circuit is done to remove noise contained in themotor current I etc. being the detected value.

The disturbance value V_(dis) being the output of the disturbanceobserver circuit 19 is fed back to a first voltage command value V_(ref)being the output of a current control circuit 11, and added thereto bythe second adding circuit 23. The back electromotive force EMF isfurther added thereto by an adding circuit 18 to compute a new secondvoltage command value (V_(ref)+V_(dis)+EMF) and input it to a PWMcontrol circuit 12. By compensating the disturbance value V_(dis)control errors generated by the factors (ii) and (iii) can becompensated. Since error due to (ii) and (iii) is considerably smallerthan error generated by the back electromotive force, it can becompensated by the disturbance observer sufficiently. The inventionsolves the disadvantage of conventional control by conducting backelectromotive force compensation separately from the disturbanceobserver circuit, the disadvantage is that it is attempted to compensateeven the back electromotive force EMF only with the disturbance observercircuit but it fails.

Therefore, nonlinear error such as modeling error of the motor,detection error, and analog-digital conversion error can be compensatedby the disturbance observer according to the second aspect of theinvention, and the back electromotive force can be compensated by thefirst aspect of the invention. Thus, in combination of the first andsecond aspects of the invention, which are features of the invention,control error caused by the nonlinear elements can be fully compensated.This effect exerts an advantage in electric power steering apparatusthat an electric power steering apparatus with small motor torqueripple, no abnormal vibrations in steering, and an excellent steeringfeeling can be realized.

Subsequently, the invention has the effect to compensate the nonlinearelement beforehand to linearize the motor for reduced control error, andthus the invention exerts the same advantage when applied to FF controlas well as FB control.

Hereinafter, an embodiment that the first and second aspects of theinvention are applied to FB control will be described with reference toFIG. 4.

The point different from the configuration of FF control shown in FIG. 3in the configuration of FB control will be described below. Thedifference is the input of the current control circuit 11 and theprocessing thereof. More specifically, the input of the current controlcircuit 11 is the motor current I detected and fed back by a currentdetection circuit 15 and the current command value I_(ref) computed by acurrent command value computation circuit 10. The current controlcircuit 11 is configured so that the motor current I and the currentcommand value I_(ref) are inputted thereto, the difference thereof iscomputed by a subtraction circuit 11-1, the difference is inputted to aproportional integral circuit 11-2, and the voltage command valueV_(ref) is outputted.

The operation of FB control having the configuration shown in FIG. 4will be described below.

The steering torque command value T_(ref) is inputted to the currentcommand value computation circuit 10 to compute the current commandvalue I_(ref), the motor current I detected by the current detectioncircuit 15 and the current command value I_(ref) are inputted to thecurrent control circuit 11. In the current control circuit 11, thedifference between the current command value I_(ref) and the motorcurrent I is computed by a subtraction circuit 11-1, the difference isinputted to the proportional integral circuit 11-2, and the voltagecommand value V_(ref) is computed as output.

Conventionally, since the voltage command value V_(ref) is directlyinputted to the PWM control circuit 12 in the motor drive circuit, theback electromotive force EMF being the nonlinear element is alsocontrolled as mixed with (R+s·L)·I of the linear element. Thus, controlerror is large, and control is unstable.

However, in the invention, the disturbance value V_(dis) being the othernonlinear element that is computed by a disturbance observer circuit 19is added by the second adding circuit 23, and a new voltage commandvalue V′_(ref) (=V_(ref)+V_(dis)) is computed to compensate thedisturbance value V_(dis). Furthermore, the back electromotive force EMFbeing the nonlinear element that is computed by a back electromotiveforce computation circuit 17 is added beforehand to the V′_(ref) by anadding circuit 18 before the input of the motor drive circuit forcompensation, and a new second voltage command value(V_(ref)+V_(dis)+EMF) is computed. Therefore, also in FB control towhich the invention is applied, the motor model is linearized to satisfythe conditions (i), (ii) and (iii) described above. For control over themotor 14 of the electric power steering apparatus, a control apparatuswith less control error and stable control can be obtained.

Accordingly, also in the electric power steering apparatus using FBcontrol to which the invention is applied, an advantage can be expectedthat an electric power steering apparatus with small motor torqueripple, no abnormal vibrations in steering, and an excellent steeringfeeling can be achieved.

In the description above, the embodiment has been described in which theLPF circuit of a first order lag circuit is used in the disturbanceobserver circuit 19 and the back electromotive force computation circuit17, but it is not limited to the first order lag function. Furthermore,the motor drive circuit is not limited to PWM control, but the sameeffects can be exerted in PAM control, chopper control, rectangular wavecontrol, etc.

Next, a third aspect of the invention having been made to achieve thesecond object of the invention is a system in which the backelectromotive force computed based on the voltage and current of themotor is used to temporarily compute a rotational speed ω and anelectrical angle θ of the motor, the electrical angle θ is added with alag Δθ computed based on the rotational speed ω, the back electromotiveforce is again computed by the electrical angle (θ+Δθ) with no lag, andthe back electromotive force with no lag is used in compensation forcontrol.

A fourth aspect of the invention having been made to achieve the secondobject of the invention is a method in which the detected backelectromotive force with a lag is partially simulated not to have a lagso that the back electromotive force with a lag computed based on themotor voltage and current is multiplied by a predetermined gain toobtain the most effective result for control that reduces torque ripplescaused by back electromotive force compensation. The principles will bedescribed in detail later with reference to FIG. 11. In the fourthaspect of the invention, it is not correct back electromotive forcecompensation with no lag in the entire area of the back electromotiveforce as the third aspect of the invention and the advantage is slightlyreduced as torque ripple control. However, it has another advantage thatis not provided by the third aspect of the invention that only gainadjustment allows high speed control processing with less arithmeticprocessing.

An embodiment of the third aspect of the invention will be describedwith reference to FIGS. 6, 7 and 8. In FIG. 6, in the basic controlexcept back electromotive force compensation of a motor 14, the currentcommand value I_(ref) is computed by a current command value circuit 10based on the steering torque command value T_(ref). To the command valueI_(ref), there are two cases: the first case is the motor currentfeedback control (hereinafter, it is denoted as FB control), and thesecond case is feed forward control (hereinafter, it is denoted as FFcontrol). However, in the embodiment, an example that it is applied toFF control will be described, but it is not limited to FF control inimplementing the invention. A current control circuit 11 computes thecommand value V_(ref) based on the command value I_(ref). In the currentcontrol circuit 11, the following is implemented:V _(ref)=(R+s·L)·I _(ref)   (8)

Next, in an adding circuit 18, the command value V_(ref) is added with aback electromotive force EMF2 which has been subjected to a lagcompensation, which will be described later. A PWM control circuit 12controls an inverter circuit 13 based on the added value (V_(ref)+EMF2),and the motor 14 is drive controlled by the output current of theinverter circuit 13.

Here, what is important is that suppose the back electromotive forceEMF2 previously referred is accurately computed as back electromotiveforce with no lag with respect to the actual back electromotive force ofthe motor 14, torque control can be also done for motor torque controlwith less ripple. Therefore, it is important how accurately the backelectromotive force EMF2 can be computed.

A computation scheme of the back electromotive force EMF2 with no lagwill be described below. First, the motor voltage V and the motorcurrent I are detected by a voltage detection circuit 16 and a currentdetection circuit 15. The motor voltage V and the current I are made asinput, and a back electromotive force EMF1 is computed by a first backelectromotive force computation circuit. In the computation scheme, thecurrent I is inputted to a transfer function 17-1 in a backelectromotive force 1 computation circuit 17 shown in FIG. 7 based onEquation (1), the output of the transfer function 17-1 is subtractedfrom the voltage V in a subtraction circuit 17-2, and then the backelectromotive force EMF1 is computed. In addition, the backelectromotive force 1 computation circuit 17 corresponds to the firstback electromotive force computation circuit. As described before, thetransfer function 17-1 has the first order lag function, causing a lag.Furthermore, in digital control, since the voltage V and the current Idetected by the voltage detection circuit 16 and the current detectioncircuit 15 are analog quantity, a lag is generated when converted todigital values. A filter in hardware before A/D conversion is a cause ofa lag in the back electromotive force EMF1.

Subsequently, the angular velocity ω and the electrical angle θ of themotor 14 are computed by a phase computation circuit 19 based on theback electromotive force EMF1. The back electromotive force and theangular velocity have a relationship expressed by Equation (9):ω=EMF/Ke   (9)where, Ke is a motor back electromotive force constant (V/rpm).

Therefore, the back electromotive force EMF1 computed by the backelectromotive force 1 computation circuit 17 is inputted to an angularvelocity computation circuit 19-1 to compute the angular velocity ω inaccordance with the relationship expressed by Equation (9).

Next, for the computation scheme of the electrical angle θ, there is anintegral relationship between the angular velocity ω and the electricalangle θ expressed by the following Equation (10):θ=θ₀ +∫ω·dt   (10)In addition, when the motor is attached with a simple rotationalposition sensor such as a Hall sensor, the value of the electrical anglecan be sensed in a discrete manner. For example, in the example of theembodiment, the detected value of the electrical angle θ₀ (=0, 60, 120,180, 240, and 300 degrees) can be detected from the Hall sensor at every60 degrees, and thus, for example, the value of the electrical anglefrom 0 degree to 60 degrees or from 60 to 120 degrees are computed byintegration by Equation (10). Accordingly, for the electrical angle θ,the angular velocity ω computed by the angular velocity computationcircuit 19-1 is inputted to an electrical angle computation circuit 19-2to compute the electrical angle θ in accordance with Equation (10).

Next, compensation of the phase lag Δθ of the electrical angle θ, whichis most important in the third aspect of the invention will be describedwith reference to FIG. 8. The lag of the electrical angle is influencedby the angular velocity ω. The lag becomes greater as the angularvelocity ω is faster. Therefore, in the embodiment, lag compensation iscompensated by the relationship expressed by Equation (11).Δθ=[(Δθ2−Δθ1)·(ω−ω1)/(ω2−ω1)]+Δθ1   (11)where, the equation is held in ω1<ω<ω2.In addition, Δθ=Δθ1, where ω<ω1, and Δθ=Δθ2, where ω2<ω.

The relationship between Δθ and ω in Equation (11) is measured byexperiments using actual devices, that is, an actual motor and an ECU.

The angular velocity ω and the electrical angle θ with a lag computed bythe phase computation circuit 19 are inputted to an adjustment circuit20. The angular velocity ω is first inputted to a phase lag computationcircuit 20-1 to compute the phase lag Δθ. Then, an adding circuit 20-2adds the phase lag Δθ of lag compensation computed by the phase lagcomputation circuit 20-1 to the electrical angle θ with a lag computedby the phase computation circuit 19, and the adjusted electrical angle(θ+Δθ) is computed. The adjusted electrical angle (θ+Δθ) expresses anaccurate electrical angle with no lag.

Subsequently, based on the accurate adjusted electrical angle (θ+Δθ),the back electromotive force is again computed. For the scheme, a backelectromotive force 2 computation circuit 21 which is a second backelectromotive force computation circuit is used for computation. Theback electromotive force 2 computation circuit 21 is formed of anormalized back electromotive force computation circuit 21-1 and arevolution correction circuit 21-2. In the normalized back electromotiveforce computation circuit 21-1, the back electromotive force EMF₁₀₀₀ ofthe motor rotating at 1000 rpm is computed based on the adjustedelectrical angle (θ+Δθ). In the revolution correction circuit 21-2, theback electromotive force can be expressed as Equation (12) because it isproportional to revolutions. For example, when the motor rotates at 1100rpm, the value computed by the normalized back electromotive forcecomputation circuit 21-1 should be multiplied by 1.1.EMF2=(ω/1000)·EMF ₁₀₀₀   (12)

Here, the normalized back electromotive force computation circuit 21-1will be described. The back electromotive force waveforms generated bythe electrical angle θ are varied depending on the types of motors ordesign values. For example, for a sinusoidal motor, the backelectromotive force waveform is sinusoidal, and for a rectangular wavemotor, it is trapezoidal. Furthermore, in order to reduce cogging torqueof the motor, design is devised in such a way that a flat portion of atrapezoidal wave is widened as much as possible. Thus, the normalizedback electromotive force computation circuit 21-1 uses an actualdesigned motor to determine the back electromotive force EMF₁₀₀₀ at 1000rpm by actual measurement. Then, the adjusted electrical angle (θ+Δθ)with no lag and the angular velocity w computed by the adjustmentcircuit 20 are inputted to the back electromotive force 2 computationcircuit 21, and then an accurate back electromotive force EMF2 iscomputed.

The computed accurate back electromotive force EMF2 is added to thecommand value V_(ref) outputted from the current control circuit 11 inthe adding circuit 18 in FIG. 6, to be a new command value(V_(ref)+EMF2). It is inputted to the PWM control circuit 12, and theinverter circuit 13 is controlled based on the command of the PWMcontrol circuit 12. Since the new command value (V_(ref)+EMF2) iscompensated with the back electromotive force EMF2 with no lag, themotor 14 can be controlled with small torque ripple. Again emphasized isthat the torque control small torque ripple can be achieved because theback electromotive force is compensated in the current control loop byaccurate back electromotive force with no lag.

Although, in FIG. 7, that the first back electromotive force computationcircuit, and the control block diagram in which the angular velocity,and the electrical angle are determined is represented in a single-linediagram, in a three phase brushless DC motor, for example, it ispossible to compute more refined back electromotive force based on thedetected values of three-phase voltage and current as shown in FIG. 9.

Furthermore, even for another type of motor such as a sinusoidal motorand a rectangular wave motor, not a brushless DC motor, the principlesof the invention can be applied to expect the advantage that torqueripples are reduced.

Moreover, in the description above, the example is taken for explanationthat the first back electromotive force computation circuit and thesecond back electromotive force computation circuit are applied to thebrushless DC motor. In the case of different types of motors, the sameadvantage can be obtained when the back electromotive force computationequation matched with that motor is used.

Besides, in FIG. 6, the example is taken for explanation that thecurrent control circuit 11 is for FF control. Even in the case of FBcontrol as shown in FIG. 10, the back electromotive force compensationexerts an advantage. In FIG. 10, the current control circuit 11 takes anFB control method in which a subtraction circuit 11-1 calculates adifference between the motor current I detected by a current detectioncircuit 15 and the command value I_(ref) computed by the current commandvalue computation circuit 10 and the difference is inputted to theproportional integral circuit 11-2 to compute the command value V_(ref).

Next, the fourth aspect of the invention will be described. The backelectromotive force EMF1 computed by the back electromotive force 1computation circuit 17 has a lag with respect to the actual motor backelectromotive force. What affect the lag has with respect to the torqueoutput is that output is always generated less than a required amountwhen torque output rises or drops. Then, the back electromotive forceEMF1 with a lag is multiplied by gain to compensate the output shortageof torque due to the lag, and then the output shortage can becompensated in some range where the back electromotive force is varied.

More specifically, even though the back electromotive force with a lagis multiplied by a predetermined gain, the entire area of the backelectromotive force cannot be the back electromotive force with no lag,but only a portion of the back electromotive force in which the lag isdifficult to be compensated by the control of the current controlcircuit etc. can be converted to the back electromotive force with nolag.

For easy understanding of this system, it will be described withreference to FIG. 11. FIG. 11(A) depicts the relationship between theback electromotive force EMF1 with a lag computed by the backelectromotive force 1 computation circuit 17 and the actual backelectromotive force. Then, FIG. 11(B) depicts the relationship betweenthe actual back electromotive force and the back electromotive forceK·EMF with gain adjustment.

In FIG. 11(B), a portion surrounded by ellipse A is the portion in whichit is difficult to reduce torque ripples by the current control circuitetc. A portion surrounded by ellipse B is the portion in which the lagof EMF can be compensated by the current control circuit as error anddisturbance. A portion surrounded by ellipse C is also the portion inwhich compensation by the current control circuit is possible. Thus,when a gain is multiplied, it is important to multiply the gain so thatthe actual back electromotive force is overlapped with the backelectromotive force K·EMF1 multiplied by the gain in the portionsurrounded by ellipse A.

An embodiment of the fourth aspect of the invention based on this ideawill be described with reference to FIG. 12. In FIG. 12, for gain K,K=1.2, for example. Therefore, the back electromotive force EMF1outputted by a back electromotive force 1 computation circuit 17 ismultiplied by 1.2 by a multiplication circuit 22, added to the commandvalue V_(ref) by an adding circuit 18. It is inputted to a PWM controlcircuit 12 as a new command value (V_(ref)+K·EMF1), and an invertercircuit 13 controls a motor 14 by the control of the PWM controlcircuit.

When the command value of the PWM control circuit 12 is (V_(ref)+EMF1),compensation by the back electromotive force is short due to a lag.Thus, torque ripple is large, and the torque output is rather small.However, when the command value is compensated to (V_(ref)+K·EMF1), itbecomes close to compensation by the accurate back electromotive forceEMF2 with no lag so that torque ripple become small, and the torqueoutput can be outputted large.

In the fourth aspect of the invention, since it is not required ofcomplicated processing as in the third aspect of the invention, it canobtain advantages that processing speed is fast, the circuit can besimplified, and the capacity of ROM etc. can be small, which are notprovided by the third aspect of the invention.

The advantages of the third and the fourth aspects of the invention willbe described with reference to FIG. 13. According to a simulation for abrushless DC motor, supposing that a desirable torque output is 100%,and a torque ripple is 0%, when the back electromotive force added bythe adding circuit 18 is only EMF1 with a lag, the torque output is 94%,and the torque ripple is 10%. On the other hand, when the backelectromotive force added by the adding circuit 18 is K·EMF1,improvement is observed that the torque output is 100%, and the torqueripple is to 8%. Furthermore, when the back electromotive force added bythe adding circuit 18 is EMF2 with no lag, the best effect can beobtained that the torque output is 100%, and the torque ripple is 6.5%.

As described above, according to the control apparatus of the electricpower steering apparatus of the first and second aspects of theinvention, by compensating the nonlinear element of the motor model ofthe electric power steering apparatus beforehand to linearize the motormodel it is possible to provide the control apparatus of the electricpower steering apparatus with less control error, stablecontrollability, small motor output torque ripple, and good wheelsteering feeling.

In addition to this, since the motor back electromotive force tocompensate the nonlinear element can be determined based on the motoroutput voltage and output current, an expensive rotational speed sensoris not required to determine the angular velocity as in the conventionalexample, and it can be advantageously implemented by a simpleconfiguration.

Furthermore, according to the control apparatus of the electric powersteering apparatus of the third and the fourth aspects of the invention,the back electromotive force with no lag is computed and the backelectromotive force is compensated in the control loop. Thus, there isthe advantage that the control apparatus of the electric power steeringapparatus with small output torque ripple of the motor, smooth steering,and less noise is provided.

1. A control apparatus of an electric power steering apparatus in whichsteering auxiliary power by a motor is given to a steering system of avehicle, the control apparatus is comprising: a motor drive circuitwhich drives said motor; a current control circuit (11) which computes afirst voltage command value that is a control command to said motordrive circuit; a back electromotive force computation circuit (17) whichcomputes a back electromotive force of said motor based on the outputvoltage and output current of said motor drive circuit; and an addingcircuit (18) which adds said back electromotive force to said firstvoltage command value to compute a second voltage command value that isa new control command to said motor drive circuit.
 2. The controlapparatus of an electric power steering apparatus according to claim 1,characterized in that: a second adding circuit (23) is disposed betweensaid current control circuit (11) and said adding circuit (18); theoutput of said current control circuit (11) is inputted to said secondadding circuit (23), and the output of said second adding circuit (23)is inputted to said adding circuit (18); a disturbance observer circuit(19) is disposed to which the outputs of said second adding circuit (23)and said motor drive circuit are inputted; and a disturbance value thatis the output of said disturbance observer circuit (19) is inputted tosaid second adding circuit (23), added to said first voltage commandvalue, and inputted to said adding circuit (18).
 3. The controlapparatus of an electric power steering apparatus according to claim 2,characterized in that: said disturbance value is a difference between avalue that is obtained by multiplying an input value of said addingcircuit (18) by a transfer function and a value that is obtained bymultiplying an output value of said motor drive circuit by the transferfunction.
 4. The control apparatus of an electric power steeringapparatus according to claim 1, characterized in that: said currentcontrol circuit (11) is a feed forward control or feedback control.
 5. Acontrol apparatus of an electric power steering apparatus in whichsteering auxiliary power by a motor is given to a steering system of avehicle, the control apparatus is comprising: a motor drive circuitwhich drives said motor; a first back electromotive force computationcircuit which computes back electromotive force (EMF1) of said motorbased on the output voltage and output current of said motor drivecircuit; a phase computation circuit which computes an electrical angle(θ) and an angular velocity (ω) based on said back electromotive force(EMF1); an adjustment circuit which computes an adjusted electricalangle (θ+Δθ) which is compensated for a phase lag (Δθ) based on saidangular velocity (ω); and a second back electromotive force computationcircuit which computes adjusted back electromotive force (EMF2) based onsaid adjusted the electrical angle (θ+Δθ).
 6. The control apparatus ofan electric power steering apparatus according to claim 5, characterizedin that: a current control circuit is provided which computes a commandvalue (V_(ref)) to control said motor based on a steering torque commandvalue (T_(ref)) to said motor, wherein said motor is controlled based ona command value (V_(ref)+EMF2) which is obtained by adding said adjustedback electromotive force (EMF2) to said command value (V_(ref)).
 7. Acontrol apparatus of an electric power steering apparatus in whichsteering auxiliary power by a motor is given to a steering system of avehicle, the control apparatus is comprising: a current control circuitwhich computes a command value (V_(ref)) to control said motor based ona steering torque command value (T_(ref)) to said motor; a motor drivecircuit which drives said motor; a first back electromotive forcecomputation circuit which computes back electromotive force (EMF1) ofsaid motor based on the output voltage and output current of said motordrive circuit; and a correction circuit which computes a corrected backelectromotive force (K·EMF1) which is obtained by multiplying said backelectromotive force (EMF1) by a predetermined value (K), wherein saidmotor is drive controlled based on a value (V_(ref)+K·EMF1) which isobtained by adding said corrected back electromotive force (K·EMF1) tosaid command value (V_(ref)).
 8. The control apparatus of an electricpower steering apparatus according to claim 6, characterized in that:said current control circuit is a feed forward control circuit or afeedback control circuit.
 9. The control apparatus of an electric powersteering apparatus according to claim 2, characterized in that: saidcurrent control circuit (11) is a feed forward control or feedbackcontrol.
 10. The control apparatus of an electric power steeringapparatus according to claim 3, characterized in that: said currentcontrol circuit (11) is a feed forward control or feedback control. 11.The control apparatus of an electric power steering apparatus accordingto claim 7, characterized in that: said current control circuit is afeed forward control circuit or a feedback control circuit.